"I just guided them about what would suit for the levels."
Underwood's school has about 200 students.
All the older students are taught from the American Jazz Dance Affiliation syllabus that Underwood owned and ran for 25 years, after inheriting it from her friend Mary "Pixie" Evers. Underwood set the syllabus, taught it and ran courses.
The technique it teaches can be applied to other dance genres, she said. It is one of the most taught dance disciplines in New Zealand, with about 50 studios, examinations and a student congress every two years.
Owning it meant keeping abreast of new developments and travelling to Australia.
Underwood has sold it, but still runs Phoenix Dance Workshops with Sharon Stevens-Cottle, bringing tutors from the United States every year.
Her average week has 18 hours of dance teaching, plus preparation.
She has had thousands of students. Some have achieved top marks in exams and made dance their career. One of them, dancer and comedian Joana Simmons, was in Whanganui and announced at the show.
Underwood loves teaching, and keeps fees for her classes low. Any profit after a show is put back into the students.
"I don't have to teach. I teach because I love it. I don't want to close the door yet, because I have still got plenty to offer," she said.
Her passion for dance started early. Growing up in Christchurch her father was a pianist and accompanist, and his piano was on the other side of her bedroom wall.
"I went to sleep every night with singers or musicians in our house and all sorts of music, and I always loved music."
At four she was watching her sister's dance classes, and she went on to learn ballet herself. She was also a gymnast.
"I was always very active."
She started teaching dance in New Plymouth - first ballet, then modern dance and then jazz dance. She moved to Whanganui in 1971, and in the early days had to choreograph a lot of dances herself.
Music still makes her want to move, and she has watched anxious students soften and relax through dance. It's good for the soul, she said.
"You can have a person who dances, and the music is on the outside of them, they're not connected. Where the dance and the music are connected - that's when the person doing the dancing becomes a dancer."
• Underwood's Moving with the Times shows are at 5pm on April 10 and 11 at the Royal Wanganui Opera House. There will be a huge range of music - lyrical and funky jazz, musical theatre, hip-hop.